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Pigs were raised on six isotopically controlled diets to examine the dietary macronutrients used in the synthesis of bulk bone biochemical components (apatite, collagen and lipids) and individual compounds (bone fatty acids, cholesterol and amino acids from collagen). δ 13 C values of apatite and bulk bone lipids reflected those of the whole diet, with 13 C apatite-whole diet = 10.2 ± 1.3‰ and 13 C bone lipids-whole diet = −2.4 ± 0.7‰. A wide variation observed in the 13 C collagen-whole diet values (0.5 to 6.1‰) was hypothesized to reflect the relative importance of (i) the direct incorporation of essential amino acids, and (ii) the balance between direct incorporation and de novo synthesis of non-essential amino acids. Linear regression (n = 6) was used to assess the relationship between the δ 13 C values of whole diet and bulk bone components and individual compounds. Whole diet δ 13 C values showed a strong correlation with those of bone cholesterol (R 2 = 0.81) and non-essential fatty acids (0.97 ≤ R 2 ≤ 0.99). Not surprisingly, bone linoleic acid δ 13 C values correlated well with dietary linoleic acid (R 2 = 0.95). Mass balance calculations using the δ 13 C values of single amino acids accurately predicted the δ 13 C value of whole collagen. The δ 13 C values of whole diet were well correlated with those of the non-essential amino acids, alanine (R 2 = 0.85) and glutamate (R 2 = 0.96) in collagen. The essential amino acids leucine ( 13 C collagen leu-diet leu = 0.5 ± 1.2‰) and phenylalanine ( 13 C collagen phe-diet phe = −0.6 ± 0.6‰) showed little isotopic fractionation between diet and bone collagen.

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